Hydraulic control system for work vehicles

ABSTRACT

A hydraulic control system for work vehicles including a control unit to control the steering wheels of a vehicle via a cylinder and a priority valve which by a first load-sensing system controls the pressure fluid flow to the control unit and to a subsequent proportional valve group. By a second load-sensing system the proportional valve group controls the pressure fluid flow to secondary hydraulic motors or work cylinders to control the work tools of the vehicle. The control system also includes an open-centre valve connection and a fixed displacement pump. The first load-sensing system is connected with a second load-sensing system, whereby drop-off of the pressure fluid flow to the control unit is avoided at simultaneous operation of the latter and of one or several of the proportional valves as is the case with known control systems. Hereby a control system is provided which is particularly agreeable to operate for the driver of the work vehicle.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic control system for workvehicles, including a control unit connected to a first hydraulic workmachine for controlling the steering wheels of the vehicle, a primarypriority valve which by means of a first load-sensing system controlsthe pressure fluid flow to the control unit and the pressure fluid flowto at least one subsequent proportional valve, which by means of asecond load-sensing system controls the pressure fluid flow to aconnected secondary hydraulic work machine for controlling one of thework tools of the vehicle, and an open-centre valve connection to a tankand a fixed displacement pump.

Hydraulic control systems of this type are very widely used. Thesecontrol systems are installed particularly in work vehicles likefork-lift trucks, tractors, agricultural, forestry and contractors'smachines.

The control unit is provided with a steering wheel with which the driverof the work vehicle directly actuates the control unit mechanically whenhe wishes to change the movement of the work vehicle. By means of one orseveral control levers the driver can actuate the proportional valves ofthe system, thereby controlling the work tools of the vehicle whenrequired. The transfer of signals from the control levers to theproportional valves may be mechanical, hydraulic, or electrical. Theprimary priority valve is designed to give the highest priority to thepressure fluid flow to the control unit. When the wheel is turned, thepressure fluid flow is divided in the priority valve, so that asufficient flow of pressure fluid is continuously led to the controlunit via the CF (control flow) port of the priority valve. The remainingflow of pressure fluid from the pump is available to the proportionalvalves via the EF (excess flow) port of the priority valve.

When neither the steering wheel nor the control levers to theproportional valves are actuated by the driver, the priority valve willbe in a non-priority position, and the pressure fluid flow from the pumpwill flow to the tank via the open-centre valve connection. When thecontrol unit is actuated via the steering wheel, the priority valve willcontrol the distribution of pressure fluid via the first load-sensingsystem, so that as the main rule the control unit will always receive asufficient flow of pressure fluid depending on the actual steering speedimparted by the driver through the wheel.

One problem of the hydraulic control systems of the type mentioned inthe introduction is the fact, however, that the pressure fluid flow tothe control unit ceases briefly at certain activation conditions whenthe control unit and one or several of the work tools of the workmachine are operated simultaneously during the operation of the workvehicle, which will be explained in more detail in the following:

If one of the proportional valves is actuated via a control leversimultaneously with the wheel being turned to one extreme position, theload-sensing system of the relevant proportional valve will cause theopen-centre valve connection to throttle down the tank connection, sothat a pressure fluid flow can be led to the proportional valve beingactuated. Hereby the pressure on the EF port of the priority valve willadjust to the pressure required by the proportional valve. It may be acase of the pressure at the CF port at the priority valve being muchlower, depending on the pressure being required by the control unit fromthe pressure fluid flow supplied.

When the operation with the relevant work tool is finished, and theproportional valve is put in neutral position via the control lever, thepressure in the load-sensing system of the proportional valve will berelieved to the tank, whereby the open-centre valve connection willrelieve immediately and open to the tank. Hereby the pressure on the EFport of the priority valve will drop off. Because the priority valvemust react as a consequence of the opening to the tank of theopen-centre valve, a time delay will occur, and a temporary "shortcircuit" will occur across the priority valve until it has had time toreact. The "short-circuit" will cause the pump pressure and thereby thepressure at the CF port of the priority valve to drop off briefly(milliseconds). When the pressure at the CF port drops off, the controlunit receives no pressure fluid flow, with the result that the wheelwill stop briefly in its activation movement as a consequence of amoment peak. The driver of the work vehicle will feel this as a briefbeat in the wheel, which is very annoying to the driver.

During operation of the work vehicles mentioned in the introduction withthe hydraulic control system in question, many times during a work daythe driver will actuate one or several of the work tools of the vehiclewhile at the same time operating the steering unit. The vehicle moves,and the driver wishes to change the path of the work vehicle. It may bea fork-lift truck, for instance, moving in a store, where the driverturns the wheel of the truck while at the same time activating aproportional valve controlling a hydraulic motor, which via a chaindrive raises or lowers a load on the fork of the truck. When the load ofthe truck has been placed at the required height above the ground, thedriver places the proportional valve in neutral position, whereby abrief beat will occur in the wheel, which will be particularly annoyingto the driver.

For a number of years it has been attempted to redress the describedproblem, for instance by adapting the pressure build-up of the priorityvalve to the control system in question, but this has not at all beenenough to remedy the problem.

SUMMARY OF THE INVENTION

The hydraulic control system according to the present invention ischaracterised in that the first load-sensing system is connected to thesecond load-sensing system. Hereby the previously mentioned beats in thewheel are avoided, when one of the proportional valves is put in neutralposition after activation of one of the work tools.

Hereby the priority valve must react only to a change corresponding tothe difference between the pressure in the first load-sensing system andthe second load-sensing system. Therefore the priority valve must travelshorter than in the previously known control system, which results in aconsiderably quicker reaction of the priority valve, which thereforeensures correct supply of pressure fluid to the steering unit, thusavoiding the beats in the wheel mentioned.

By putting the proportional valve in neutral position, the pressure inthe second load-sensing system will be relieved to tank, whereafter thepressure in the first load-sensing system of the steering unit will takeover. Consequently, the open-centre valve connection will not relieveand open up to the tank, as in the case of previously known hydrauliccontrol systems, but will adapt to the same pressure level as in thefirst load-sensing system. The pressure at the open-centre valveconnection will be transmitted to the EF port of the priority valve andfurther on to the P port of that valve, to which port the pump isconnected in such a manner that the pressure mentioned is immediatelyavailable also at the CF port of the priority valve. Therefore the valvemust react only to the previously mentioned pressure difference betweenthe pressure in the first load-sensing system and the secondload-sensing system.

The hydraulic control system includes an especially compact modulegroup, in which both the first and the second load sensing systems areintegrated.

When it is desirable to give the pressure fluid flow to a predeterminedproportional valve a first secondary priority after the pressure fluidflow to the control unit, at least one module including a secondarypriority valve is inserted in the proportional valve group.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in the following with reference tospecially preferred embodiments and the drawing, where

FIG. 1 shows schematically a first embodiment of the hydraulic controlsystem, and

FIG. 2 schematically another embodiment of the control system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the control system shown schematically in FIG. 1, several of theinput valves are drawn schematically with a view to simplifying andfacilitating the understanding of the control system design. Thus onlythe parts of the valves required for understanding the invention areshown.

The control system 1 shown in FIG. 1 includes a control unit 2 connectedto a first hydraulic work machine such as a hydraulic cylinder 3 forcontrol of the steering wheels of the vehicle (not shown). The controlsystem 1 further includes a priority valve 4, which includes a CF portthrough which the pressure fluid flow (control flow) to the control unit2 is supplied, and an EF port through which the pressure fluid flow(excess flow) is supplied to an open-centre pump side module 5.

The pump side module 5 includes an open-centre valve connection 6 to atank 7. The control system also includes a fixed displacement pump 8,which supplies a pressure fluid flow to the P port of the priority valve4.

In the embodiment shown, the hydraulic control system 1 also comprisesthree proportional valves 9a, 9b, 9c, which are designed as separatemodules combined in a proportional valve group. The proportional valvegroup 9a-9c is mounted with the pump side module 5.

To each of the ports a, respectively b, of the proportional valves 9a,9b, 9c, a secondary hydraulic work machine may be connected. Thus to theproportional valve 9a a hydraulic motor 10 is connected, which drivesfor instance the chain drive for height adjustment of the load forks ofa fork lift truck. To other proportional valves 9b, 9c may be connectedhydraulic cylinders 11, 12, which in the case of the fork lift truckcontrol the angle to horizontal of the load forks.

In the control system according to the present invention, all knowntypes of proportional valves 9a-9c may be used, and therefore only theinput parts of the proportional valve 9a are shown in full, with thereservation that parts of the pipe system have been omitted which arenot required for understanding the invention. Thus the proportionalvalve 9a includes a main valve 14a, which determines magnitude anddirection of the pressure fluid flow to the motor 10, a pressurecompensator valve 15a, which compensates for variations in load andsystem pressure, thereby keeping the pressure drop across the main valve14a constant, a non-return valve 16, which prevents the oil fromunintentionally running back from the motor 10, and a 2-way valve 17a,which ensures that the system pressure is always controlled by thepressure in the highest loaded valve section.

The mode of operation of the hydraulic control system according to thepresent invention is explained in detail in the following with referenceto the example, where the control system is mounted in a fork lifttruck.

The control unit 2 is provided with a steering wheel connected at 18,and when the fork lift truck driver turns the wheel, the pressure fluidflow is divided in the priority valve 4 so that a sufficient pressurefluid flow is continuously fed to the control unit 2 via the CF port.The remaining pressure fluid flow from the pump is available to the pumpside module 5 of the proportional valve group 9a-9c via the EF port ofthe priority valve 4. The distribution of the pressure fluid isregulated by a first load sensing system LS1 so that the pressure fluidflow to the control unit 2 is always determined by the actual controlvelocity resulting from the actuation of the steering wheel by thedriver.

In the embodiment shown, the proportional valve group 9a-9c includes acommon, second load-sensing system LS2, which mutually connects the2-way valves 17a-17c of the proportional valves with the valve 6 in theopen-centre pump side module 5.

According to the present invention the first load-sensing system LS1 isconnected with the second load-sensing system LS2 via the conduit 19.

By turning the steering wheel and thereby operating the control unit 2the pressure in the first load-sensing system LS1 will influence thepriority valve 4 to the effect that, as mentioned, the control unit 2 issupplied with the required pressure fluid flow. At the same time thepressure in the second load-sensing system LS2 will influence the 2-wayvalves 17a-17c in the proportional valve group 9a-9c so that the valve 6in the pump side module 5 adapts to the same pressure, which may be forinstance 90 bar.

At simultaneous operation of one or several of the proportional valvessuch as the proportional valve 9a, the valve will require supply of apressure of for instance 200 bar, which will thereby be adapted to thesecond load-sensing system LS2.

Thereby the 2-way valve 17a will change position so that the 90 barpressure from the first load-sensing system LS1 is disconnected, and thevalve 6 will then throttle to the required 200 bar. The 200 bar pressurewill be transmitted through the EF port of the priority valve 4 to thepump 8. Thus the pressure at the CF port of the priority valve 4 will beinfluenced very briefly by the pressure rise at the P port of the pump,and the priority valve 4 will then throttle until the pressure at the CFport corresponds again to the pressure in the first load-sensing systemLS1. The priority valve 4 travels sufficiently to ensure that thecontrol unit 2 is continuously supplied with the correct pressure fluidflow, and it is therefore a matter of a so-called pressure compensation.

When the proportional valve 9a is subsequently set in a neutralposition, for instance when the driver sets the valve control lever atneutral after the motor 10 has lifted or lowered the load forks of thefork lift truck to the required height while driving, the pressure inthe second load-sensing system LS2 will be relieved to tank 7. The 2-wayvalve 17a in the proportional valve 9a will then change position againso that the pressure in the first load-sensing system LS1 of the controlunit will be induced in the second load-sensing system LS2 via theconduit 19. This causes the valve 6 to adjust to the 90 bar, which willbe transmitted to the EF port of the priority valve 2 so that the 90 barpressure is immediately available to the control unit 2.

Thus the priority valve 4 must react to a change from 200 bar to 90 baronly, which allows the priority valve to travel shorter as seen inrelation to its travel in known systems, which results in quickerreaction of the priority valve, which again ensures a correct pressurefluid flow to the control unit 2. This prevents the annoying briefpressure drop-off at the CF port, and thereby drop-off of the pressurefluid supply to the control unit, as it is the case in the knownhydraulic control systems, and which caused the "beats" of the steeringwheel that were so very unpleasant for the driver. Surprisingly it hasproved that the hydraulic control system according to the presentinvention always ensures the required pressure fluid supply to thecontrol unit at simultaneous actuation of the control unit and placingin neutral of the proportional valves actuated. This ensures the workvehicle driver a much more pleasant operation of the control unit whendriving.

In FIG. 2 is shown a particularly suitable embodiment of a hydrauliccontrol system 21 according to the present invention, where the priorityvalve is built into a module being part of a module group. The modulegroup includes an open-centre pump side module 22a, a subsequentpriority module 22b for the control unit, and subsequent proportionalvalve modules 22c and 22d for secondary hydraulic work motors orcylinders.

The pump side module 22a is connected with the fixed-displacement pump23 and includes a valve connection 24 to tank 25. The priority module22b includes a priority valve 26, which by means of a first load-sensingsystem LS1 controls the pressure fluid supply to a control unit 27 and ahydraulic cylinder 28 for control of the steering wheels of the vehicle.In the embodiment shown, each proportional valve module 22c, 22dincludes a main valve 29c, 29d, a combined pressure compensator andnon-return valve 30c, 30d, and a 2-way valve 31c, 31d in a common secondload-sensing system LS2. The proportional valve modules 22c, 22d may beconnected to hydraulic cylinders 32c, 32d for operation of work tools onthe work vehicle.

In the hydraulic control system 21 the load-sensing systems LS1 and LS2are directly connected at the transitions between the adjacent modules22a-22d. The module group 22a-d is particularly simple and easy toproduce and at the same time simple and quick to mount in a workvehicle.

In the module design described, high flexibility is achieved at the sametime, because it is easy to replace or move a module, for instance if itis desirable to place one more proportional valve module in the groupfor control of one more tool. At the same time it will be possible toplace extra priority valve modules in the module group, if a previouslydetermined priority sequence is required for the pressure fluid flow tothe proportional valve modules. A priority valve module inserted betweenthe priority valve module 22b and the proportional valve module 22c willensure that the pressure fluid flow to the proportional valve module 22creceives the first secondary priority after the pressure fluid flow tothe control unit 27.

The invention is described with reference to particularly preferredembodiments. Many changes may be made, however, without deviating fromthe very principle of the invention. Thus the input valves of thecontrol system may be designed and combined into modules in manydifferent ways as long as the first and second load-sensing systems areconnected.

I claim:
 1. A hydraulic control system for a work vehicle, including acontrol unit connected to a first hydraulic work machine for controllingsteering wheels of the vehicle, a primary priority valve having a firstload-sensing system, said first load-sensing system having means tocontrol pressure fluid flow to the control unit and pressure fluid flowto at least one subsequent proportional valve, and including a secondload-sensing system having means to control pressure fluid flow to aconnected secondary hydraulic work machine for controlling a work toolof the vehicle, and including an open-center valve connection to a tankand a fixed displacement pump, the first load-sensing system beingconnected to the second load-sensing system for joint operation of theprimary priority valve.
 2. A hydraulic control system according to claim1, in which all the load sensing systems of the subsequent proportionalvalves are connected to form a common, second load-sensing system.
 3. Ahydraulic control system according to claim 1, in which the proportionalvalves are located in separate modules, which are combined into aproportional valve group.
 4. A hydraulic control system according toclaim 3, in which the proportional valve group is combined with anopen-centre pump side module.
 5. A hydraulic control system according toclaim 4, in which the primary priority valve is located in a modulewhich is located between the pump side module and the proportional valvegroup.
 6. A hydraulic control system according to claim 3, in which atleast one module including a secondary priority valve is located in theproportional valve group.